The present invention relates to an exhaust gas recirculation control device of an engine.
Conventionally, exhaust gas recirculation (EGR) control devices of engines are known. For example, JP2007-292028A discloses such an exhaust gas recirculation control device, which includes an exhaust turbocharger, a low-pressure EGR passage, a low-pressure EGR valve, and an exhaust shutter valve (exhaust throttle valve). The exhaust turbocharger has a turbine disposed in an exhaust passage of the engine and a compressor disposed in an intake passage of the engine. The low-pressure EGR passage connects a part of the exhaust passage downstream of the turbine to a part of the intake passage upstream of the compressor. The low-pressure EGR valve is disposed in the low-pressure EGR passage and changes a cross-sectional area of the low-pressure EGR passage. The exhaust shutter valve is disposed in a part of the exhaust passage downstream of the connected part to the low-pressure EGR passage, and changes a cross-sectional area of the exhaust passage. In JP2007-292028A, the control device also includes a high-pressure EGR passage connecting a part of the exhaust passage upstream of the turbine to a part of the intake passage downstream of the compressor, and a high-pressure EGR valve disposed in the high-pressure EGR passage and for changing a cross-sectional area of the high-pressure EGR passage.
In JP2007-292028A, an amount of exhaust gas recirculated from the engine by the low-pressure EGR passage is adjusted by controlling openings of the low-pressure EGR valve and the exhaust shutter valve. In this case, when the recirculation amount of exhaust gas is small, the exhaust shutter valve is fully opened and the opening of the low-pressure EGR valve is controlled to adjust the recirculation amount of exhaust gas. When the low-pressure EGR valve is fully opened and the recirculation amount cannot be increased anymore, the exhaust shutter valve is narrowed while the low-pressure EGR valve is fully opened, so as to increase the recirculation amount.
As disclosed in JP2007-292028A, when the recirculation amount of exhaust gas is small, by fully opening the exhaust shutter valve and controlling the low-pressure EGR valve to adjust the recirculation amount of exhaust gas, an exhaust pressure loss of the turbine can be reduced as much as possible, and a fuel consumption can be maintained low.
Here, when a transmission of a vehicle in which the engine is installed is in a low-speed gear range (e.g., a first or second gear range), an engine speed changes greatly with respect to an accelerator position, and a target recirculation amount also changes greatly according to the change in the engine speed. In this case, if the exhaust shutter valve is fully opened, a difference between pressures on the exhaust passage side and the intake passage side of the low-pressure EGR valve in the low-pressure EGR passage is comparatively small. Therefore, while the target recirculation amount changes greatly, in order to achieve the changed target recirculation amount by the low-pressure EGR valve, an opening of the low-pressure EGR valve needs to be changed greatly, which causes a problem that a follow-up responsiveness of the opening of the low-pressure EGR valve with respect to the change in the target recirculation amount degrades.